Despite the availability of numerous potent antibiotics and modern life-support therapies, bacterial infections are still a major cause of morbidity and mortality, especially in hospital patients, elderly, and immunocompromised. Moreover, at least 18 bacteria could be used in bioterrorism attack or biological warfare and pose a significant potential threat to public health. The exposure from such an attack may involve contaminated air, objects, food, or water, and the agent may not be initially known or may be a mixture of several bacteria. Therefore, in all these groups an immediate and broad-based protection against many bacteria would be most beneficial in early stages of host defense to prevent or stop the infection at the portal of entry (such as skin, eyes, gastrointestinal tract, and upper respiratory tract). Because of broad specificity of innate immunity for many bacteria, innate immunity is likely to be the most effective first line of defense to combat such bacterial infections immediately after the initial exposure. Thus, enhancing host antibacterial innate immunity at the site of contact with bacteria could prevent establishment of infection or complement other therapies, and save lives in epidemics, biological warfare or bioterrorism attacks, or in other exposures to bacteria that are likely to cause serious infections. One group of human bactericidal proteins that could be used for prevention of such infections is peptidoglycan recognition proteins (PGLYRPs). Because PGLYRPs are naturally produced on human skin, on mucous membranes, and in the sweat, sebum, and saliva, they can be developed into useful agents that can effectively protect people from infections with bacteria. The long-term goal of these studies will be to develop PGLYRP molecules with high and broad spectrum of activity into clinically applicable agents that can be used to enhance host defenses against bacterial infections. The goal of this application is to determine the mechanism through which PGLYRPs kill bacteria. The specific aims of this application are: (a) to determine the morphologic site of action of PGLYRPs in bacteria and exact mechanism of their bactericidal activity, including testing the hypotheses that they inhibit peptidoglycan synthesis in the bacterial cell wall or lyse bacteria by an enzymatic hydrolytic mechanism;(b) to define the specificities of PGLYRPs and requirements for their activity;and (c) to determine whether bacteria may become resistant to PGLYRPs and to identify bacterial genes responsible for resistance. This proposal will determine how human antibacterial proteins, called peptidoglycan recognition proteins, kill bacteria. This knowledge will help to develop these proteins into clinically applicable agents that in the future can be used to enhance people's defenses against bacterial infections. New antibacterial agents are needed because of the increasing resistance of bacteria to commonly used antibiotics.